Photocells for use in light measuring devices such as those utilized in the photographic and optical fields employ either photovoltaic or photoresistive elements which are responsive to incident light either by producing a voltage potential or by changing their resistance, respectively. Generally the output signals of such elements vary as the intensity of the incident light in accordance with some function which is utilized either to drive a meter or perform some function. So far as known, prior photocells include opaque elements in the assemblies of their components. Such assemblies block part of the optical path which they intercept and hence are normally offset from the optical systems they control or are arranged to monitor. Alternately they are inserted into the optical path and swung out of such path when the optical system is to be utilized.
A photocell which is permanently arranged offset from an optical system must be small so as not to occupy too much space and additionally cannot give an absolutely true reading of the light passing into the optical system along its axis. A structure which must move into and out of the path of the optical system is complex and expensive.
Known photo cells are structures independent of optical systems and must be constructed, handled and assembled as another component of the system.
There has been a need for a photocell which will not substantially absorb the light which is incident thereon. Such a photocell would have to be sensitive and accurate. It could advantageously be incorporated directly into an optical system with accompanying saving in space and elimination of structural components.
The expression "thin film" as used herein means a layer or coating not more than a few thousands of Angstroms. Actually, the invention is concerned with thin layers of the order of substantially less then 1000 A.